As an organic material for use as a proton conductive material, there have been reported polymers belonging to so-called cation exchange resin, such as polystyrenesulfonic acid, polyvinylsulfonic acid, perfluorosulfonic acid polymer, or perfluorocarboxylic acid polymer (cf. Polymer Preprints, Japan, Vol. 42, No. 7, p 2490-2492(1993), Polymer Preprints, Japan, Vol. 43, No. 3, p 735-736(1994), and Polymer Preprints, Japan, Vol. 42, No. 3, p 730(1993)). Such organic material provides an advantage capable of easily forming a membrane on a substrate or directly onto an electrode from a polymer solution, and is already used in practice as a solid electrolyte of a high energy density, for example, in a fuel cell for a rocket (for example Nafion). Although the organic materials show a high proton conductivity at a room temperature in a water-containing condition, they have a drawback that the conductivity tends to decrease at a high temperature of 100° C. or higher.
Japanese Patent Application Laid-Open No. 2001-114834 discloses a polymer electrolyte using a polymer of acrylate ester compound having phosphate group. Such a membrane basically constituted of organic main chains has drawbacks of low heat resistance and low barrier property to hydrogen gas or methanol used as a fuel.
Japanese Patent Application Laid-Open No. 2000-90946 discloses a proton conductive membrane made of an electrolyte which comprises a composite of an inorganic material and an organic material bonded by urethane bond in nano-level and a phosphate compound introduced thereto as a conductivity-providing agent. This membrane can function in a high temperature condition.
Also Japanese Patent Application Laid-Open No. 2002-184427 discloses a proton conductive material having a siloxane network with a sulfonic acid functional group. Introduction of such an inorganic component improves the heat resistance of the membrane, but there still remains a drawback of low water resistance.
Also Japanese Patent Application Laid-Open No. H02-110121 discloses a UV-curable composition constituted of acrylorganosiloxane, polyacrylate or polymethacrylate, a phosphate-containing acryl monomer and a photopolymerization initiator. This composition gives a film having satisfactory adhesiveness to a substrate material, but cannot be used as an electrolyte membrane because it has little proton conductivity.
The present invention has been made in consideration of the aforementioned prior arts, and is to provide an electrolyte membrane of siloxane-based polymer having a high proton conductivity in the wide ranges of temperature and humidity and excellent in resistance to water and methanol. It also is to provide a solid polymer fuel cell utilizing such an electrolyte membrane.